This invention relates to improvements in a method of and apparatus for displaying an image of phase contrast in a scanning transmission electron microscope (STEM).
In general, an image of phase contrast in a scanning transmission electron microscope is obtained with a method in which a small aperture is provided between a specimen and a detector so as to narrow a detecting angle.
FIG. 1 is a diagram showing the outline of the method. In case of illuminating a specimen 2 (hereinafter, a phase object having a grating-like structure shall be considered as an ideal specimen) with a primary electron beam 1, electron beams transmitted through the specimen 2 can be divided into scattered electron beams 3 generated by scattering within the specimen 2 and a transmitted electron beam 4 transmitted through the specimen 2 without interacting therewith. Each scattered electron beam 3 involves a phase shift relative to the transmitted electron beam 4 on account of the scattering within the specimen. The phase contrast causes the scattered electron beam 3 and the transmitted electron beam 4 to interfere when they overlap each other. It is a part 5 that is subjected to the interference. Since information on the phase contrast is obtained through the interference phenomenon, the variation of the intensity of the electron beam in the interfering region 5 needs to be detected in order to form an image of phase contrast. To this end, there has been adopted an expedient in which an aperture plate 6 is inserted into the interfering region 5 to confine a narrow detecting angle and to enhance the coherence, whereupon a signal is obtained. This expedient has realized optically the same condition as that in the case of forming an image of phase contrast in a transmission electron microscope (TEM). The intensity of the electron beam having passed through an aperture 6' of the aperture plate 6 is detected by a detector 7. The detected signal is amplified by an amplifier 8, and is supplied for the intensity modulation of a CRT (cathode-ray tube) 9. On the other hand, the scanning on the screen of the CRT 9 is synchronous with the scanning of the primary electron beam which is effected by a power supply for scanning 10 and deflection coils 11 (in the figure, the arrow 12 indicates the direction of the scanning on the specimen 2). Therefore, an image including the information of the phase contrast is formed on the screen of the CRT 9. This is the so-called image of phase contrast.
However, the image of the phase contrast acquired with this method is of the superposition between the contrast based on the phase shift (phase contrast) and the contrast (bright field image) ascribable to the decay of the electron-beam intensity occurring when the primary electron beam is transmitted through the specimen. It is not an image which consists only of the information of the phase shift.
As an expedient for improving the drawback, there has been proposed a method in which only the phase contrast is emphasized to display an image (refer to Optik, vol. 41, p. 452-456, 1974). This method is characterized in that two detectors are used to construct a new detector system without disposing the aperture plate 6.
FIG. 2 is a top plan view of the detector system in this method. Detectors (I) and (II) have their detecting faces arranged so as to lie in contact with each other through a straight line l within an identical plane. When they are installed at the position of the detector 7 in FIG. 1, the spots of the electron beams transmitted through the specimen become as shown by three circles in FIG. 2. In the figure, numeral 14 designates the spot formed by the transmitted electron beam, numerals 13 and 13' indicate the spots formed by the scattered electron beams, and hatched parts A and B are the interfering regions. With the detector system thus constructed, the difference between the electron-beam intensities detected by the respective detectors (I) and (II) is taken. Then, since the bright field images are isotropic, they are canceled, and only the difference (A-B) of the electron-beam intensities in the interfering regions A and B can be derived as a signal. Accordingly, an image of phase contrast formed by the use of this signal consists only of the information of the phase shift.
With such detector system, however, the two detectors are required, which gives rise to such disadvantages (1) that the coadjustments between the detectors are necessary, (2) that the mechanical butt of the detecting faces of the two detectors needs to be precise, and (3) that since the symmetry with respect to an optical axis is necessary, the axial alignment is necessitated, resulting in complicated operations.